32 THE ELEMENTS OF PLANT FOOD 



till it turns a milky white, calcium carbonate having 

 been thus produced by the CO 2 in the breath. The 

 reaction is : 



Ca(OH) 2 + CO 2 = CaCO 3 + H 2 



The decomposition of carbonaceous matter in the 

 soil forms CO 2 . This uniting with H 2 O forms a weak 

 acid, carbonic acid, CO 2 + H 2 O = H 2 COa. This weak 

 acid acts on the minerals in the soil and renders 

 the plant food bound up in them available. The 

 minerals dissolved by this acid are much greater than 

 would be dissolved by pure water, and since the for- 

 mation of carbonic acid is continuous, the amount of 

 mineral matter made available is very great in the 

 aggregate. The carbonic acid combines readily with 

 certain bases to form compounds beneficial to plant 

 growth. 



The most important use of CO 2 is its direct use as 

 food for plants. All green vegetation is feeding on 

 the CO 2 and building up carbonaceous tissue. This 

 feeding takes place in the presence of green matter 

 (chlorophyll) in vegetation under the influence of sun- 

 light. CO 2 breaks up; O is thrown out into the air 

 and C is combined with H 2 O to make some of the or- 

 ganic compounds, such as starch, sugar, and woody 

 fiber. 



The food of man and other animals comes directly 

 or indirectly from plants. Much of the food thus taken 

 is oxidized in the blood and tissues, and CO 2 is formed 

 and again returned to the air. Thus the cycle of 

 changes continues, and the amount of CO 2 in the air is 

 held unchanged in amount. 



Humus. Animal and vegetable matter in soil 



